A schematic picture would help, but I believe you want to move the wheels back. If you know the excess weight on the rear axle (and the max allowed weight ) and have a rough idea of the dimensions and load distribution it is possible to get an estimate of how much further back the wheels need to be.

I was under the assumption that if we simply moved the wheels as far back as they could go, that that would solve the problem. The problem is, when the truck is loaded, it picks up product at different locations, with different weights each time. And I DON"T want to leave it at the driver's discretion as to how far forward or backward to keep that rear axle.

Moving the wheels as far back as possible is the best you can do. However, that does not mean you've solved the problem for any load/distribution. It is probably a good idea to figure out what you max allowable load should be for that position of the axle.